An anodization method is a method for forming an oxide film on the surface of a metal used as an anode in an acid solution or a neutral solution. This method is frequently used for forming oxide films on valve metals such as, aluminum and tantalum. For example, with aluminum, porous thick oxide films are formed in an acid solution of sulfuric acid, oxalic acid, phosphoric acid, or the like and barrier-type dense thin films are formed in a neutral solution of a borate, a phosphate, an adipate, or the like. Porous aluminum oxide films are used for corrosion prevention, friction prevention, coloring decoration, and the like, and barrier-type films are widely used as dielectrics of electrolytic capacitors.
An electrolytic capacitor generally includes an anode composed of a valve metal such as aluminum or tantalum, a dielectric composed of an oxide film formed on the surface of the anode, and a cathode formed to hold an electrolyte between the cathode and the dielectric. In the electrolytic capacitor, the driving electrolyte has two important functions. One of the two is the function as the actual cathode. Namely, the electrolyte functions to extract a capacitance from the dielectric formed on the anode and is required to have high electric conductivity, i.e., high electron conductivity. The other is the function to protect and repair a very thin oxide film, i.e., the chemical function to form a new oxide in a defect of an aluminum or tantalum oxide film on the basis of the ion conductivity possessed by the electrolyte. Namely, the anodization is used for forming a dielectric oxide film in an electrolytic capacitor and for repairing a defect of an oxide film. Therefore, the electrolyte of the electrolytic capacitor is required to have an anodizing ability.
As the electrolyte of the electrolytic capacitor, an organic solvent such as ethylene glycol or γ-butyrolactone containing an organic acid, an inorganic acid, or a salt thereof is generally used. Specific examples of an organic acid, an inorganic acid, or a salt thereof added to the solvent include phosphoric acid, formic acid, acetic acid, ammonium adipate, ammonium succinate, tertiary amines, and quaternary ammonium salts. Such a composite electrolytic system is used for forming an electrolyte having excellent ion conductivity (Patent Document 1).
Although the conductivity of such a liquid electrolyte is improved by adding the above-described additive, the conductivity is only about 10−3 S/cm, which is unsatisfactory for realizing a low-impedance capacitor. Also, the liquid electrolyte causes a dry-up phenomenon due to evaporation of the solvent used, and both the anodizing property and conductivity are lost by the dry-up. Therefore, the electrolyte is unsatisfactory in the long-term life and heat resistance.
In order to improve these properties, use of a molten salt as an electrolyte for a capacitor has been investigated. For example, an investigation has been conducted for forming an electrolyte for a capacitor by melting or melting and then solidifying an electrolytic salt having a nitrogen-containing heterocyclic cation having a conjugated double bond or a nitrogen-containing heterocyclic ring containing a conjugated double bond without using a solvent (Patent Document 1).
Also, an investigation has been conducted for forming a capacitor including an electrolyte for an electrolytic capacitor interposed singly or together with a separator between an anode foil and a cathode, the electrolyte being prepared by melting a mixture of a carboxylate and a carboxylic acid without using a solvent (Patent Document 2). However, these electrolytes are solid at room temperature and thus have the very low anodizing ability and low conductivity. Therefore, the electrolytes have been not yet put into practical applications.
On the other hand, solid capacitors not containing a solvent have been recently developed. Specifically, these capacitors each include, as an electrolyte, a conductive polymer, such as polypyrrole, polyaniline, or a polythiophene derivative. Since these conductive polymers have extremely higher electric conductivity (electron conductivity) than that of the above-described conventional electrolytic solutions each containing an electrolyte and a solvent, the internal impedance of a capacitor using such a conductive polymer as an electrolyte can be decreased. In particular, when these conductive polymer capacitors are used for high-frequency circuits, excellent properties are exhibited. Therefore, such conductive polymer capacitors are establishing an important position in the electrolytic capacitor market.
However, conductive polymers basically do not have ion conductivity, and thus conductive polymer capacitors are far inferior to conventional capacitors each including an electrolytic solution in the anodizing function to repair oxide films of electrolytic capacitors. It is generally said that in a conductive polymer capacitor, a conductive polymer present on the dielectric surface of a damaged portion is insulated by the de-doping reaction of the conductive polymer due to the Joule heat generated in damage to a dielectric film, thereby preventing the breakage of the dielectric film. Such a mechanism is fundamentally different in principle from a mechanism occurring in the function to repair an oxide film of a conventional capacitor using an electrolytic solution (Non-patent Document 2).
Consequently, the conductive polymer capacitors are disadvantageous in that capacitors with a high withstand voltage cannot be formed. Specifically, under present conditions, when aluminum is used for an anode, a conductive polymer capacitor having a withstand voltage up to only about 16 V can be produced, for example, with a formation voltage of 70 V, and when tantalum is used, a conductive polymer capacitor having a withstand voltage up to only about 12 V can be produced, for example, with a formation voltage of 34 V. The term “formation voltage of 70 V” means that in forming a dielectric oxide film on a valve metal surface, the DC voltage applied to the valve metal, i.e., the formation voltage, is 70 V. Of course, the withstand voltage can be basically increased by increasing the formation voltage. In this case, however, the capacitance of a capacitor decreases as the formation voltage increases, and the withstand voltage does not increase in proportion to increases in the formation voltage. Therefore, this is not said to be a preferred method.
As an attempt to improve the withstand voltage characteristic of a conductive polymer capacitor, an electrolytic capacitor is disclosed, in which an electrolyte including a conductive polymer and an organic acid onium salt is used (Patent Document 3). However, it is assumed that the organic acid onium salt is basically a solid salt. Therefore, in order to improve the withstand voltage, the ratio between the conductive polymer (A) and the organic acid onium salt (B) is thought to be preferably in a range of (A):(B)=1:0.1 to 5 and more preferably in a range of (A):(B)=1:0.2 to 2. However, at a ratio in this range, the withstand voltage is certainly improved, but the conductivity characteristic degrades, undesirably resulting in the deterioration in the impedance characteristic of the capacitor. Apart from the above-described technique relating to electrolytic capacitors, a molten salt in a liquid state at room temperature have been developed and attracted attention. The molten salt is referred to as an “ionic liquid”, includes a combination of a quaternary salt cation, such as imidazolium or pyridinium, and an appropriate anion (Br−, AlCl−, BF4−, or PF6−), and frequently contains a halogen. The ionic liquid has the properties such as nonvolatility, noninflammability, chemical stability, high ion conductivity, and the like, and attracts attention as a reusable green solvent used for various syntheses and chemical reactions such as catalytic reaction. However, there has been no example of investigation of the ionic liquid from the viewpoint of the anodizing property, i.e., from the viewpoint of the formation of an oxide film on a valve metal surface or the repair of an oxide film.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-13278    Patent Document 2: Japanese Unexamined Patent Application Publication No. 5-101983    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-22938    Non-patent Document 1: Denkai Chikudenki Hyoron (Electrolytic Condenser Review), Vol. 53, No. 1, p. 101 (2002)    Non-patent Document 2: Denkai Chikudenki Hyoron (Electrolytic Condenser Review), Vol. 53, No. 1, p. 95 (2002)